IRMPD Spectroscopy: Evidence of Hydrogen Bonding in the Gas Phase Conformations of Lasso Peptides and their Branched-Cyclic Topoisomers.

Lasso peptides are natural products characterized by a mechanically interlocked topology. The conformation of lasso peptides has been probed in the gas phase using ion mobility-mass spectrometry (IM-MS) which showed differences in the lasso and their unthreaded branched-cyclic topoisomers depending on the ion charge states. To further characterize the evolution of gas phase conformations as a function of the charge state and to assess associated changes in the hydrogen bond network, infrared multiple photon dissociation (IRMPD) action spectroscopy was carried out on two representative lasso peptides, microcin J25 (MccJ25) and capistruin, and their branched-cyclic topoisomers. For the branched-cyclic topoisomers, spectroscopic evidence of a disruption of neutral hydrogen bonds were found when comparing the 3+ and 4+ charge states. In contrast, for the lasso peptides, the IRMPD spectra were found to be similar for the two charge states, suggesting very little difference in gas phase conformations upon addition of a proton. The IRMPD data were thus found consistent and complementary to IM-MS, confirming the stable and compact structure of lasso peptides in the gas phase.